1. Field of the Invention
The present invention relates to a multilayer printed circuit board of the type which comprises a base member, at least one inner buildup layer formed on the base member, and an outer buildup layer formed on the inner buildup layer for mounting electronic components. The present invention also relates to a method of making such a multilayer printed circuit board.
2. Description of the Related Art
FIG. 3 illustrates an example of prior-art multilayer printed circuit board. The illustrated multilayer printed circuit board Y includes a core member 70 having a first surface 70A and a second surface 70B. The first surface 70A is formed with a first base wiring pattern 70a, whereas the second surface 70B is formed with a second base wiring pattern 70b. Two buildup layers 71, 73 are disposed on the first surface 70A, and other two buildup layers 72, 74 are disposed on the second surface 70B. Each of the buildup layers 71xcx9c74 as a surface 71A, 72A, 73A, 74A formed with a buildup wiring pattern 71a, 72a, 73a, 74a. 
The base member 70 is formed with a via hole 8A in which conductor portion 9A is provided. The first base wiring pattern 70a and the second base wiring pattern 70b are electrically connected to each other through the conductor portion 9A. Similarly, the buildup layers 71xcx9c74 are formed with via holes 8Bxcx9c8H respectively provided with conductor portions 9Bxcx9c9H, through which the base wiring patterns 70a, 70b and the buildup wiring patterns 71axcx9c74a are electrically connected. Various kinds of electronic components (not shown) may be mounted on the outermost buildup layers 73, 74 of the multilayer printed circuit board Y.
The above-described multilayer printed circuit board Y may be manufactured through the successive steps shown in FIGS. 4Axcx9c4L.
First, as shown in FIG. 4A, a base member 70 is prepared which has a first surface 70A and a second surface 70B respectively formed with conductor layers 70axe2x80x2, 70bxe2x80x2 made of copper for example. Then, as shown in FIG. 4B, a via hole 8A is formed in the base member 70 by drilling for example. Subsequently, as shown in FIG. 4C, a conductor portion 9A is formed in the via hole 8A by electroless plating for example. After that, the conductor layers 70axe2x80x2, 70bxe2x80x2 are subjected to etching, thereby providing base wiring patterns 70a, 70b, as shown in FIG. 4D.
Then, as shown in FIG. 4E, buildup layers 71, 72 each having a surface 71A, 72A formed with a conductor layer 71axe2x80x2, 72axe2x80x2, are bonded to the first and the second surfaces 70A, 70B of the core member 70, respectively. Subsequently, in a manner similar to the foregoing, via holes 8B, 8C, 8D are formed as shown in FIG. 4F, conductor portion 9B, 9C, 9D are formed as shown in FIG. 4G, and buildup wiring pattern 71a, 72a are formed as shown in FIG. 4H. Thus, buildup wiring patterns 71a, 72a and the base wiring patterns 70a, 70b are electrically connected through the conductor portions 9Bxcx9c9D. Then, similarly to the above, buildup layers 73, 74 are laminated, and via holes 8E, 8F, 8G, 8H, conductor portions 9E, 9F 9G, 9H, and buildup wiring pattern 73a, 74a are formed, as shown in FIGS. 4Ixcx9c4L.
In the above-described multilayer printed circuit board Y, all of the buildup layers 71xcx9c74 are formed of a same material. For example, for providing a light multilayer printed circuit board Y, the buildup layers 71xcx9c74 maybe formed of a resin coated copper foil (RCC foil). On the other hand, for providing a multilayer printed circuit board Y having a high rigidity (flexural rigidity), the buildup layers 71xcx9c74 may be formed of a resin reinforced by a glass fiber such as a glass epoxy resin.
However, since the light multilayer printed circuit board Y formed of a RCC foil does not have a sufficient rigidity, it may warp when electronic components are mounted on the buildup wiring patterns 73a, 74a. Thus, it may be impossible to appropriately mount electronic components on such a light multilayer printed circuit board Y.
On the other hand, the rigid multilayer printed circuit board Y formed of a resin reinforced by a glass fiber has a disadvantageously large weight. Further, it is difficult to form via holes 8Axcx9c8H by laser beam application in such a rigid multilayer printed circuit board.
Moreover, the via holes 8Axcx9c8H formed by laser beam application generally have a diameter of 30xcx9c200 xcexcm, and it is difficult to fill the via holes with the conductive material by electroless plating in forming the conductor portions 9Axcx9c9H. Therefore, the buildup wiring patterns 73a, 74a include recesses at the via holes 8Excx9c8H where electronic components cannot be mounted. As a result, there exists a great limitation on the mounting positions of electronic components. To solve this problem, the via holes need to be filled with a conductor paste for example, which is troublesome. Further, the mounting of electronic components on the via holes filled with a conductor paste is not preferable in view of the conduction reliability of the electronic components.
It is therefore an object of the present invention to provide a light but yet rigid multilayer printed circuit board on which electronic components can be appropriately mounted.
Another object of the present invention is to provide a multilayer printed circuit board which can be made at a relatively low cost.
A further object of the present invention is to provide a method for conveniently making such a multilayer printed circuit board.
In accordance with a first aspect of the present invention, there is provided a multilayer printed circuit board comprising: a base member provided with a base wiring pattern formed on at least one surface thereof; at least one inner buildup layer laminated on said at least one surface of the base member and having a surface formed with an inner buildup wiring pattern, the inner buildup wiring pattern being electrically connected to the base wiring pattern through at least one via formed in said at least one inner buildup layer; and an outer buildup layer laminated on said surface of said at least one inner buildup layer and having a surface formed with an outer buildup wiring pattern, the outer buildup wiring pattern being electrically connected to the inner wiring pattern through at least one via formed in the outer buildup layer; wherein said at least one inner buildup layer is made of a resin material which is not reinforced by glass fibers, the outer buildup layer being made of a resin material reinforced by glass fibers.
With such a structure, the multilayer printed circuit board has a high rigidity and is unlikely to warp because the outer buildup layer is formed of a resin material reinforced by glass fibers. As a result, it is possible to appropriately mount electronic components on the board.
On the other hand, the inner buildup layer is formed of a resin material which is not reinforced by glass fibers. Therefore, it is possible to realize a weight reduction of the multilayer printed circuit board as a whole.
In the above-described structure, two or more inner buildup layers may be interposed between the outer buildup layer and the base member. In such a case, at least one of the inner buildup layers needs to be made of a resin material which is not reinforced by glass fibers, though it is more preferable if all of the inner buildup layers are made of such a material.
Preferably, the inner buildup layer may be made of a thermosetting resin such as phenolic resin, epoxy resin or polyimide resin. On the other hand, the outer buildup layer may be formed of a resin material obtained by reinforcing the above-described thermosetting resin by glass fibers. The glass fibers may be in the form of a plain-woven glass cloth or a nonwoven fabric.
The base member may be made of any insulating material. However, similarly to the inner buildup layer, the base member is preferably made of a resin material which is not reinforced by glass fibers for effectively reducing the weight of the multilayer printed circuit board.
The base wiring pattern and each of the buildup wiring patterns may be provided by forming a conductor layer on the base member or each buildup layer, forming thereon a mask, and partially etching away the conductor material. The conductor layer is formed by attaching a metal foil such as a copper foil onto the base member or each buildup layer, or by growing a metal film by plating or vapor deposition.
Each via hole may be made by drilling, laser beam application, plasma etching or photolithography to have an inner diameter of 30xcx9c200 xcexcm for example.
In the case where the via hole is to be formed by laser beam application, it is preferable that the outer buildup layer be formed of a glass fiber cloth prepreg obtained by impregnating open glass fiber cloth with a thermosetting resin. An open glass fiber cloth has a smaller fiber density at the crossing points of warps and wefts than an ordinary glass fiber cloth. Therefore, it is possible to make a hole even at the crossing points of warps and wefts by laser beam application, as opposed to an ordinary glass fiber cloth wherein it is difficult to form a hole at the crossing points of warps and wefts where the fiber density is very high.
Preferably, the multilayer printed circuit board may further comprises: an additional base wiring pattern formed on another surface of the base member; at least one additional inner buildup layer laminated on said another surface of the base member and having a surface formed with an additional inner buildup wiring pattern, the additional inner buildup wiring pattern being electrically connected to the additional base wiring pattern through at least one via formed in said at least one additional inner buildup layer; and an additional outer buildup layer laminated on said surface of said at least one additional inner buildup layer and having a surface formed with an additional outer buildup wiring pattern, the additional outer buildup wiring pattern being electrically connected to the additional inner wiring pattern through at least one via formed in the additional outer buildup layer; wherein said at least one additional inner buildup layer is made of a resin material which is not reinforced by glass fibers, the additional outer buildup layer being made of a resin material reinforced by glass fibers.
With such a structure, since the base member and the inner buildup layers (including the additional inner buildup layer or layers) are sandwiched between both outer buildup layers, the multilayer printed circuit board can enjoy a further enhanced rigidity. Since both of the two outer layers have a high rigidity, it is possible to prevent the multilayer printed circuit board from warping in any direction.
Preferably, the or each via of the or each inner buildup layer may comprise a filled via.
Further, the or each via of the or each outer buildup layer may comprise a filled via.
Preferably, the or each via of the or each inner buildup layer may comprise a filled via, while the or each via of the or each outer buildup layer may also comprise a filled via which is stacked on the filled via of the or each inner buildup layer.
Preferably, the base member may have another surface formed with an additional base wiring pattern, and the or each via of the or each inner buildup layer comprises a skip via penetrating through the or each inner buildup layer and the base member into electrical connection with the additional base wiring pattern.
According to a second aspect of the present invention, there is provided a method of making a multilayer printed circuit board, the circuit board comprising: a base member provided with a base wiring pattern formed on at least one surface thereof; at least one inner buildup layer laminated on said at least one surface of the base member and having a surface formed with an inner buildup wiring pattern, the inner buildup wiring pattern being electrically connected to the base wiring pattern through at least one via formed in said at least one inner buildup layer; and an outer buildup layer laminated on said surface of said at least one inner buildup layer and having a surface formed with an outer buildup wiring pattern, the outer buildup wiring pattern being electrically connected to the inner wiring pattern through at least one via formed in the outer buildup layer; said at least one inner buildup layer is made of a resin material which is not reinforced by glass fibers, the outer buildup layer being made of a resin material reinforced by glass fibers; wherein said at least one via of of said at least one inner buildup layer and said at least one via of the outer buildup layer is formed by the steps of: forming a via hole by laser beam application; and depositing a conductive substance in the via hole:
The deposition of the conductive substance may be performed by plating the inner surface of the via hole. Though electroless plating is generally used, pulse plating is preferable.
In pulse plating, positive and negative potentials are alternately applied to an object to be plated. When positive potential is applied, a metal film grows, whereas the metal film is partially removed when negative potential is applied. The amount of film growth is larger on a rough surface than on a smooth surface, whereas the amount of film removal is larger on a smooth surface than on a rough surface. Therefore, pulse plating makes it possible to form a metal film intensively on a rough surface. Since the inner surface of the via hole made by laser beam application is generally rough, it is possible to intensively plate the inner surface of the via hole by utilizing pulse plating. Thus, when a via hole has an inner diameter of 30xcx9c200 xcexcm and a depth of 0.05xcx9c0.5 mm for example, it is possible to fill the via hole with a conductive substance by pulse plating, thereby providing a filled via.
In this way, the via hole formed in the outer buildup layer can be filled by pulse plating without the need for performing an additional troublesome step for filling the via hole with a conductor paste for example. Therefore, it is possible to mount an electronic component even on the via hole with a connection reliability. As a result, an electronic component may be mounted at any portion of the wiring pattern. 11. The method according to claim 8, wherein the via hole has a diameter of 30xcx9c200 xcexcm.
Preferably, the outer buildup layer may be formed by laminating a prepreg of a glass-fiber-reinforce thermosetting resin on the inner buildup layer and then heating the prepreg.
Other features and advantages of the present invention will become clearer from the detailed description give below with reference to the accompanying drawings.